The present invention relates to satellite communication systems. More particularly, the present invention relates to a system and method that allows a satellite to process multiple spread-spectrum uplink channels, for example, Code Division Multiple Access (CDMA) channels, without substantially increasing the required satellite hardware.
CDMA is a technique used to communicate large amounts of information contained in many user signals simultaneously and at low power levels using a large bandwidth. CDMA is generally considered a "spread-spectrum" technology. In operation, CDMA spreads the information contained in a particular user signal over a much greater bandwidth than the original user signal. Starting from a rate of approximately 9.6 Kilobits per second (9.6 Kbps), for example, CDMA may spread user signals to a transmitted rate of approximately 1.23 Megabits per second. In accomplishing spreading, CDMA applies orthogonal codes to the data bits associated with user signals. The resulting coded data bits may then be transmitted along with the user signals of all the other users without appreciable interference by merging the coded data bits with a pseudo-random noise (PN) sequence. When the composite signal is received, the orthogonal code applied to a particular user signal is removed in a process called "despreading" that separates the user signal out from the composite signal and returns the user signal to its original rate.
The military is a traditional user of spread spectrum technology. Because the user signal is spread out along a wide bandwidth, it is very difficult to jam, difficult to interfere with, and difficult to even discover that the user signal is in the air. Due to the low transmission power levels and spreading effect of the digital codes, the composite signal appears as nothing more than a slight rise in the existing "noise floor". Other technologies, for example, Time Division Multiple Access (TDMA) tend to concentrate large amounts of information and power in a small portion of bandwidth, thus making the transmission much easier to detect and interfere with.
CDMA technology is also applied in the digital mobile telephone market and offers numerous advantages for its users. Among these advantages are that the apparent capacity of a CDMA system may be 8 to 10 times larger than that of traditional analog cellular systems, such as Advanced Mobile Phone Service (AMPS) and 4 to 5 larger than that of a digital cellular system such as the Global System for Mobile communications (GSM) system. A CDMA system may also enjoy improved call quality and enhanced privacy as well as provide longer battery life for portable phones.
CDMA is poised to take a major role in future communications systems. After first entering commercial service in Hong Kong in 1995, CDMA is now in commercial service in, for example, the United States, Korea, Canada, India, and China.
Along with the need to communicate large amounts of information is the need to communicate that information via satellite. Satellites provide wide ranging coverage of large parts of the planet and can easily send signals where it is impossible or uneconomical to place physical conductors like copper wire or fiber optic cable.
Today, however, satellite uplinks, may consist of hundreds of simultaneous users, particularly if the uplinks carry CDMA transmissions. A single satellite may have, for example, 30 or 40 uplink transponders (essentially receive antennas), each able to accept an uplink beam with a bandwidth of 250 MHz. The resultant total uplink data path would then have a capacity of nearly 8 to 10 gigabits per second.
CDMA is capable of generating massive amounts of data for an uplink beam to carry. Transmission systems may, for example, divide the 250 MHz uplink beam bandwidth into lower bandwidth uplink channels (twenty 12.5 MHz uplink channels, for example). Each uplink channel may then use CDMA techniques to carry data channels for hundreds of users. In order for a conventional satellite to despread and decode the data channel for each user, the satellite would have to carry hundreds (perhaps thousands) of sets of heavy, complex decoding electronics, and generate enormous amounts of power. Additional circuitry, of course, leads to a corresponding increase in required power and satellite size.
Increasing the size, weight, and onboard power of a satellite so that it can decode more data channels drives up the cost of the satellite dramatically. Not only does the satellite itself become more expensive because of the additional circuitry and solar panels used to provide onboard power, but it also costs more to launch the satellite because larger rockets and more propellant are required to put the satellite into orbit. Satellite size, weight, and power restrictions thereby prohibit the satellite from handling the large numbers of data channels that modern communications techniques can generate.
Thus a need is present in the industry for an improved communications system, which overcomes the disadvantages discussed above and previously experienced.